1. Field of the Invention
The invention relates generally to well logging using nuclear magnetic resonance (NMR) instruments. More specifically, the present invention relates to methods and apparatus for NMR well logging based on carbon-proton J-coupling.
2. Background Art
Oil and gas exploration and production are very expensive operations. Any knowledge about the formations that can help reduce the unnecessary waste of resources in well drilling will be invaluable. Therefore, the oil and gas industry have developed various tools capable of determining and predicting earth formation properties. Among different types of tools, nuclear magnetic resonance (NMR) instruments have proven to be invaluable. NMR instruments can be used to determine formation properties, such as the fractional volume of pore space and the fractional volume of mobile fluid filling the pore space. General background of NMR well logging is described in U.S. Pat. No. 6,140,817.
Nuclear magnetic resonance is a phenomenon occurring in a selected group of nuclei having magnetic nuclear moments, i.e., non-zero spin quantum numbers. When these nuclei are placed in a magnetic field (Bo, “Zeeman field”), they each precess around the axis of the Bo field with a specific frequency, the Larmor frequency (ωo), which is a characteristic property of each nuclear species (gyromagnetic ratio, γ) and depends on the magnetic field strength (Bo) effective at the location of the nucleus, i.e., ωo=γBo.
Proton is the major nucleus of investigation in well logging NMR applications because of its good NMR sensitivity and its high abundance in water and hydrocarbons. Furthermore, due to downhole limitations, the current well logging tools only measure T1, T2 relaxation times and diffusion effects.
In other fields of NMR applications such as chemistry, biology, and petroleum fluid analysis, proton and carbon chemical shift and J-coupling spectroscopic techniques are routinely used to determine molecular structures. Chemical shift is the term given to describe the screening effect of the electrons to the magnetic field that a nucleus experiences. Different chemical groups, such as CH2 and CH3, have different magnitude of screening effects, and, therefore, they appear as separate peaks in the proton chemical shift spectrum. The separation in frequency of different peaks is proportional to the static magnetic field strength, i.e., magnetic, field dependent. J-coupling, also known as spin-spin or scalar coupling, originates from spin interaction between nuclei through bonding electrons. See, E. L. Hahn, and D. E. Maxwell, Spin echo measurements of nuclear spin coupling in molecules, Physical Review 88, 1070-1084 (1952). J-coupling experiment is seldom performed by itself. Instead, J couplings are always measured together with chemical shifts through one-dimensional or multi-dimensional spectroscopic techniques.
As noted above, chemical shift is magnetic field dependent. The homogeneity of a static magnetic field has to be within a few parts per million (ppm) to perform chemical shift spectroscopy. This level of homogeneity is hard to realize in the wellbore using existing technologies. In contrast, J-coupling constants are independent of static magnetic field strengths and temperatures. This makes it possible to perform J-coupling experiment without chemical shift spectroscopy in inhomogeneous static and radio-frequency (RF) magnetic fields.
U.S. Pat. No. 6,111,409 issued to Edwards discloses methods for performing chemical shift spectroscopy in the wellbore. Because the static magnetic field homogeneity is difficult to achieve in the formation, the methods of Edwards involve withdrawing fluids into a formation tester before NMR measurements. Even in the formation tester, the stringent homogeneity required for conventional chemical shift measurements is not an easy task. A permanent magnet and shim coils will be used to generate the static magnetic field. First of all, the space in the tool that can accommodate the magnet and shim coils is very limited in shape and size. To achieve a 1 ppm homogeneity over a reasonable volume is a daunting task for magnet design and manufacturing. If the homogeneous volume is too small, the small sample may not be a good representation of the fluid being investigated. Secondly, temperature change can affect the strength and homogeneity of the magnetic field.
U.S. Pat. No. 6,346,813 issued to Kleinberg discloses a variety of NMR measurements for characterizing fluid samples withdrawn from subsurface formations. This patent is assigned to the assignee of the present invention and is hereby incorporated by reference. Proton and carbon. NMR measurements are among the proposed techniques.
U.S. patent application Ser. No. 10/064,529 filed by Speier on Jul. 24, 2002, discloses methods for J-spectroscopy experiments using spin-echo difference techniques. This application is assigned to the assignee of the present invention and is hereby incorporated by reference. This approach provides convenient methods to obtain J couplings in the wellbore. Although this approach is less sensitive to magnetic field inhomogeneity, it still suffers from field inhomogeneity to some extent. The inhomogeneous, B0 (static) field makes the effective B1 (radio-frequency) field inhomogeneous. As a result, the π pulses may not be accurate throughout the region of investigation. Inaccurate π pulses may diminish the J-modulation signals.
Therefore, it is desirable to have NMR methods and apparatus for determining J coupling that are less affected by magnetic field inhomogeneity.